Geochemical exploration for mineral resources typically relies on the collection of soils, sediments, or rocks and the subsequent analysis of these media for the element being sought or its pathfinders. Michaels and Riese (1986) suggested that micro-organisms, especially bacteria, should also be considered as exploration tools. Like other organisms, bacteria are sensitive to the toxic effects of heavy metals in their environment. Unlike many organisms, however, bacteria possess genetic mechanisms which allow them to adapt relatively rapidly to the presence of these metals and thereby survive and thrive in their presence (Bopp et al., 1983; Chen et al., 1985; Foster, 1983; Haefeli et al., 1984; Robinson and Touvinen, 1984; Silver, 1981; Summers, 1984). Measurement of this genetically encoded resistance can be accomplished via culture techniques which were also outlined by Michaels and Riese (1986). If the metals exerting the selection pressure requiring resistance to develop are supplied to the environment by a slowly weathering ore deposit or alteration halo, the presence of resistant bacteria may then be used as an indicator of that deposit.
While the use of microorganisms as a prospecting tool will realize a significant saving in both time and money if culture "assay" methods are employed as compared to conventional geochemical methods, culture assay methods require 5-7 days of incubation time as well as facilities for media preparation and the disposal of cultures. Culture assays are also particularly unsuited for field exploration, particularly in rugged country where seasonal access in very short. Further, as pointed out in Michaels and Riese (1986) cultures must not be incubated at temperatures above 20 degrees centigrade if the bacteria's natural environment does not exceed that temperature. Thus, the time involved to hike back out to sophisticated laboratory facilities might be sufficiently long that the bacteria collected in the samples may increase, thus distorting results or rendering the results so suspect as to be worthless. Even more importantly, the lag in obtaining assay results may mean that rapid follow-up in the field is impossible as the field crew may have moved on, and may not be able to return to the area until a year later if the prospecting season has closed or the crews are scheduled elsewhere.
Attempts to date to utilize microbes in petroleum exploration programs have relied on the detection of microbes specifically adapted to utilization of methane as a food source. The assumption is made that hydrocarbon reservoirs leak, that these escaped hydrocarbons make their way to the surface (seafloor) in some way, and are there utilized by the resident microbial community as a food source. Hydrocarbons may also represent a toxic substance to bacteria, however, and the resident microbial community may adapt to these in a manner similar to that described for metals.
Accordingly, there is a need for a fast, field manageable assay that employs the principal of microorganism sensitivity to the toxic effects of heavy metals and/or hydrocarbons in their environment, either directly as a mapping tool for such heavy metals, or as pathfinders for other metals or petroleum.